Papers by Author: M.L. Craus

Abstract: New La0.54Ho0.11Sr0.35Mn1-xCuxO3 manganites were obtained by sol-gel method using oxides and acetates and sintered finally in air at 12000 C for 15 h. The samples were studied by X-ray diffraction, magnetic and electric methods. Space group, lattice constants, average size of the crystalline blocks and positions of cations/anions in the unit cell, were determined using the FullProf code. The samples contain only a perovskite phase with an orthorhombic structure (Pnma space group). Small-angle neutron scattering (SANS) measurements were performed on the SANS-1 setup at the FRG-1 reactor of the GKSS Research Centre (Geesthacht, Germany). The sizes of magnetic clusters have been determined at room temperature.

Abstract: Transport phenomena in manganites (ABO₃), and indirectly the transition temperature from metallic to insulator state (T_MI), can be controlled by hole doping and the average ionic size at A positions (). In agreement with Zener theory, the strength of double exchange is determined by the Mn-O length (d_MnO) and Mn-O-Mn angles () of the Mn-O-Mn bonds. We will investigate the influence of substitution of Mn with V on the crystalline structure and transport characteristics in La_0.54Ho_0.11Sr_0.35Mn_1-xV_xO₃ manganites. The samples were prepared by sol-gel method to improve the purity and homogeneity of the samples. By XRD it was established that the samples contain only ABO₃ perovskite phases, except the samples with x0.1. The resistance of the samples vs temperature was determined by four probes method. The specific magnetization was obtained by using a Foner type magnetometer, working at 1.4 T, between 77 and 400 K.

Abstract: La1-x(Sr, Ca)xMnO3 manganites present a special interest due their high Curie and metal-insulator transition temperatures and the possibility to obtain magnetoresistive sensors able to work near room temperature. We have synthesized two rows of manganites, with La0.54Ho0.11Sr0.35-xKxMnO3 and La0.54Ho0.11Ca0.35-xNaxMnO3 chemical compositions. XRD analysis, performed with a Huber diffractometer at room temperature, indicated that the manganites contain perovskite phases, with Pnma structure. The atomic positions, lattice parameters and some microstructure parameters, as average size of coherent blocks and microstrains, were obtained by using GSAS program. The dependence of the resistivity with temperature and the applied magnetic field was determined by means of four probes method, between 77 and 300 K. The influence of K/Na concentration on the disorder degree (at atomic and crystallite level) and on the transport mechanisms was discussed.